The present invention relates to a combustion air supply system for a recovery furnace.
Particularly in recovery furnaces, so-called soda furnaces, designed for processing the waste liquor, so-called black lye, produced in certain manufacturing processes of paper industry, there often occur problems relating to operation, emissions and processes resulting from a combustion air supply system which has been a long-standing problem in the art. In the combustion or firing chambers of a recovery furnace designed with traditioanl combustion air supply systems, the combustion-air flows coming particularly through so-called secondary air inlet ports from all the walls of a combustion chamber on substantially the same horizontal plane, join together in the corner regions of a combustion chamber to form powerful diagonal flows directed towards the center of the combustion chamber. These flows directed from the corners towards the center of a combustion chamber merge into each other at the center of the combustion chamber and produce a powerful flow directed upwards in the center of a combustion chamber. On the basis of experiments it has been found that the rate of this local flow can be more than 15 m/s, which is a rate or velocity approximately four times higher than the average upward velocity of flue gases in a combustion chamber. Since black lye is delivered into a combustion chamber by injection in vertical direction above secondary-air ports, it is obvious that some of the black lye in the form of droplets is entrapped in the upward-directed powerful flue-gas flow which carries the droplets to the upper section of the combustion chamber and to its overhead superheaters. The droplets burn completely out, thus causing within this region too high a temperature, cloggings, corrosion and sulphur (SO.sub.2, H.sub.2 S) emissions higher than normal. It is also know that, as some of the fuel burns out "in a wrong place", temperature in the lower section of a combustion chamber, wherein the fuel is supposed to burn out completely, will be lower than it would be if the processing advanced in a desired manner so that the entire amount of black lye would end up in in a caronization layer formed in the bottom section of a combustion chamber. This naturally lowers the efficiency of the processing or the reduction of sulphur.